> It is true that Zulip writes a tiny UserMessage table row with (user_id, message_id, flags) for every active recipient of a message, we need to do so in order to track the unread state for all of those recipients (as well as related details, like mobile push notifications state). We need to write to that row a second time when the user reads the message.

No software architect here so I might be completely off the target but, is not read/unread count something that squares perfectly with the "eventually consistent" model? You don't need to write it down to the persistent storage right now as long as the client UI has it, then some kind of cache has it (so mobile can be in sync when refreshed) and then you persist it to the DB.